In the fabrication of microelectronic components, for example, semiconductor chips and semiconductor chip packaging substrates, increase in performance is generally achieved by reducing the image size of the electronic devices on the chips and by reducing the width and spacing of the electrical conductors of the wiring planes of the semiconductor chip and semiconductor chip packaging substrate. Reduced image size and spacing is achievable by using improved optical systems to project higher resolution images.
Generally, an optical system for fabricating an electronic component has a mask containing a pattern at the input of the optical system through which light of a preselected frequency is passed. The optical system contains a component, typically a lens to reduce the size of the patterns in the mask which is projected as a reduced image onto the surface of an electronic component. A reduction system is used to reduce the width and spacings of the patterns on the mask. The image is typically projected onto a resist material on the surface of the electronic component. The light projected onto the resist material causes a chemical change therein which either renders the exposed regions soluble or insoluble with respect to the un-exposed regions of the resist. The soluble regions are removed by exposing the resist to a solvent leaving a pattern which is either the positive or negative image of the mask reduced in size.
The reducing system is used to reduce the size of the patterns on the mask and to demagnify imperfections in the structure. The use of an optical system for producing a reduced image of the mask introduces distortions referred to as aberrations which are inherent in the optical components of the system.
Prior art reduction systems employed in tools known as steppers use a series of lenses to reduce the mask image and to correct the various commonly known aberrations of an optical system. However, the applicants have discovered that by using a curved mirror to provide the predominant fraction of the reducing power of the optical system, the inherent aberrations of the optical system can be corrected more effectively and with fewer optical components.
The preferred curved surface is a concave spherical mirror. A problem with using a spherical mirror in place of a lens for reducing the mask size is that the projected image of the mask is reflected back from the mirror towards the direction of the mask. Such an image cannot be easily used since the substrate on 7hich the image is to be projected must be placed in the path of the optical beam which is incident on the spherical surface. This effectively prevents a useful image from being formed using the full field of the mirror. Therefore, such systems typically use reflecting surfaces to split the mirror field of view, into a field for the object being focused and a field for the substrate on which the image of the object is focused. In such systems the size of the substrate object being imaged is constrained since the object and the image can only occupy one half the total field of the focusing mirror. The field of the focusing mirror is that region of an object or image field, over which the mirror, in conjunction with the remainder of the optical system, can properly form images.
Applicants have discovered that by using an appropriate beam splitting surface the output beam can be directed away from the input beam so that the output beam can be used to project an image onto a substrate.
The beam transmitted through and reflected from the beam splitting surface must be substantially free of distortion, aberration and apodization as a result of passing through or being reflected from the beam splitting surface. Beam splitting surfaces suitable for the optical systems of the present invention are described in U.S. patent application Ser. No. 07/185,187 filed on Apr. 22, 1988 entitled "Thin Film Beam Splitter Optical Element For Use In An Image-Forming Lens System" to A. E. Rosenbluth, the teaching of which is incorporated herein by reference.
U.S. Pat. No. 4,444,464 to Minott describes a catadioptric optical system having two symmetrically aligned off-axis Schmit optical objectives. Incident light is reflected from two primary spherical mirrors off the axis of the incident light. The light from each mirror is reflected from a beam splitter which separates the light into a plurality of spectral bands.
U.S. Pat. No. 4,694,151 describes a mirror free auto focus system having a half mirror prism and a sensor capable of detecting light in conjunction with the half mirror prism inserted between the front lens and rear lens group of the lens system.
U.S. Pat. No. 4,311,366 describes an image focusing system free of curved mirrors for use in a reprographic camera containing a flat mirror or prism or roof prism for folding an input beam. The input beam and output beam pass through lens combinations providing focusing and aberration correction.
U.S. Pat. No. 4,265,529 describes a view finder for a camera including an input lens, a flat reflecting mirror, a roof type pentagonal mirror and an eye piece.
U.S. Pat. No. 3,536,380 describes a 1X catadioptric projection system for semiconductor chip photolithographic applications. Light is directed through a lens, through a mask, onto a half silvered mirror from which it is reflected through a lens onto a concave mirror from which it reflects onto the target substrate.
U.S. Pat. No. 4,387,969 described an optical system having an objective lens group, a beam splitting prism which deflects light at a right angle through a lens to focus an image on a film.
U.S. Pat. No. 2,166,102 describes a telescope having an objective lens and using a prism or a plain mirror to reflect light at an angle to a concave mirror.
U.S. Pat. No. 3,001,448 describes a system using a beam splitter for correcting astigmatism produced by a shallow dome by introducing positive stigmatism by using rotating prism.
U.S. Pat. No. 4,742,376 describes a step and repeat system which uses a Dyson-Wynne catadioptric projection system.
U.S. Pat. No. 4,743,103 describes a lens system for a photographic printer which rotates the image through 90 degrees without effecting the inversion needed in a printer lens.
It is an object of this invention to provide an optical projection system with an extended field which will faithfully reproduce submicron geometries over a large substrate area.
It is another object of this invention to provide a substantially telecentric reduction catadioptric relay lens with diffraction limited performance over the ultraviolet bandwidth, most preferably of an excimer laser.
It is another object of the invention to exploit the very sensitive deep UV resists and highly intense excimer laser beams for the optical microlithography for microelectronic integrated circuits by sacrificing net transmittance of the optical system which is a consequence of employing the beam splitting technique to form an accessible and useful image.
It is another object of this invention to extend the limits of optical microlithography to quarter micron resolution by employing a high numerical aperture with partially coherent illumination of the mask improving the recordability of the image due to the resulting enhancement of contrast beyond the limit of 49% for incoherent 248 nm illumination in the case of an aberration free in focus lens at a numerical aperture of 0.6.
These and other objects, features and advantages will be apparent from the following more particular description of the preferred embodiments and the figures appended thereto.